Police were 'blocked in' by a large group of youths while trying to search a man who was believed to be carrying a knife in Leeds city centre.
Officers were assaulted and spat at by a crowd who gathered on Lands Lane while they were detaining a young man whom a member of the public claimed to have seen showing off a weapon.
One witness, who filmed the altercation, claimed 'around 200 guys' were involved in the disorder outside trainer store Foot Asylum on Boxing Day afternoon.
Police had to request back-up.
Paul Young, from Pudsey, said:
"We turned up just as it was kicking off. There were about 200 guys and about 30 police. It lasted around 45 minutes and was right outside Foot Asylum - it looked at first like it might have been a dispute between gangs."
Police confirmed that the suspect was found not to be carrying a knife.
West Yorkshire Police said:-
"At 4.14pm yesterday (26/12) police were contacted by a member of the public reporting having seen a youth in Lands Lane showing a knife to a group of other youths before concealing it in his waistband.
"Officers attended and detained a suspect but were blocked in by a large group and had to request urgent assistance. Officers reported being assaulted, including being spat at.
"Other officers responded to the incident and the crowd was dispersed at about 4.45pm.
"A dispersal order was put in place to deter any further anti-social behaviour and officers patrolled the area to reassure the public.
"The male who was detained was searched and not found to have knife on him."